Plague is considered a vector-borne illness, meaning that it requires a living host to carry it from one organism to another. The bacteria is carried by fleas and the small rodents. The Y. pestis enters the fleas’ intestines, then enters a new host when the flea has its next blood meal and regurgitates the bacteria afterwards. While in the fleas’ intestines, the bacteria loses its capsular layer, making it easy for the human body to defeat. However, once it has entered the human body, it is able to suppress the natural inflammatory response so that nothing appears wrong at first. But soon, the bacteria are taken up by tissue macrophages, which protect the bacteria from the body’s natural defenses. While inside the tissue macrophages the bacteria have time to synthesize their virulence factors, and when they have finished, they kill the tissue macrophages and emerge back into the human body whereupon they spread fairly quickly to the lymph glands. Once leaving the macrophages, the bacteria can spread to the bloodstream and once there, it is able to spread throughout the entire body, effectively invading their host.